1. Field of the Invention
The present invention relates to a thermally-assisted magnetic recording head fabricated by joining a slider and a light source unit that includes a light source, and further relates to a method for manufacturing the thermally-assisted magnetic recording head.
2. Description of the Related Art
As the recording densities of magnetic recording apparatuses become higher, as represented by magnetic disk apparatuses, further improvement has been required in the performance of thin-film magnetic heads and magnetic recording media. As the thin-film magnetic heads, a composite-type thin-film magnetic head is widely used, which has a stacked structure of a magnetoresistive (MR) element for reading data and an electromagnetic transducer for writing data.
Whereas, the magnetic recording medium is generally a kind of discontinuous body of magnetic grains gathered together, and each of the magnetic grains has a single magnetic domain structure. Here, one record bit consists of a plurality of the magnetic grains. Therefore, in order to improve the recording density, it is necessary to decrease the size of the magnetic grains and reduce irregularity in the boundary of the record bit. However, the decrease in size of the magnetic grains raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume.
As a measure against the thermal stability problem, it may be possible to increase the magnetic anisotropy energy KU of the magnetic grains. However, the increase in energy KU causes the increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. Whereas, the intensity of write field generated from the thin-film magnetic head is limited almost by the amount of saturation magnetic flux density of the soft-magnetic material of which the magnetic core of the head is formed. As a result, the head cannot write data to the magnetic recording medium when the anisotropic magnetic field of the medium exceeds the write field limit.
Recently, as a method for solving the problem of thermal stability, so-called a thermally-assisted magnetic recording technique is proposed. In the technique, a magnetic recording medium formed of a magnetic material with a large energy KU is used so as to stabilize the magnetization, then anisotropic magnetic field of a portion of the medium, where data is to be written, is reduced by heating the portion; just after that, writing is performed by applying write field to the heated portion.
In this thermally-assisted magnetic recording technique, there has been generally used a method in which a magnetic recording medium is irradiated and thus heated with a light such as near-field light. In this case, it is significantly important to stably supply a light with a sufficiently high intensity at a desired position on the magnetic recording medium. However, from the beginning, more significant problem to be solved exists in where and how a light source with a sufficiently high output of light should be disposed inside a head.
As for the setting of the light source, for example, U.S. Pat. No. 7,538,978 B2 discloses a configuration in which a laser unit including a laser diode is mounted on the back surface of a slider, and US Patent Publication No. 2008/0056073 A1 discloses a configuration in which a structure of a laser diode element with a monolithically integrated reflection mirror is mounted on the back surface of a slider. Further, US Patent Publication No. 2005/0213436 A1 discloses a structure of slider that is formed together with a semiconductor laser, and Robert E. Rottmayer et al. “Heat-Assisted Magnetic Recording” IEEE TRANSACTIONS ON MAGNETICS, Vol. 42, No. 10, p. 2417-2421 (2006) discloses a configuration in which a diffraction grating is irradiated with a light generated from a laser unit provided within a drive apparatus.
As described above, various types of the setting of the light source are suggested. However, the present inventors propose a thermally-assisted magnetic recording head with a “composite slider structure” which is constituted by joining a light source unit provided with a light source to the end surface (back surface) of a slider provided with a write head element, the end surface being opposite to the opposed-to-medium surface of the slider. The “composite slider structure” is disclosed in, for example, US Patent Publication No. 2008/043360 A1 and US Patent Publication No. 2009/052078 A1. The advantages of the thermally-assisted magnetic recording head with the “composite slider structure” are as follows:
a) The head has an affinity with the conventional manufacturing method of thin-film magnetic heads because the opposed-to-medium surface and the element-integration surface are perpendicular to each other in the slider.
b) The light source can avoid suffering mechanical shock directly during operation because the light source is provided far from the opposed-to-medium surface.
c) The light source such as a laser diode and the head elements can be evaluated independently of each other; thus the degradation of manufacturing yield for obtaining the whole head can be avoided; whereas, in the case that all the light source and head elements are provided within the slider, the manufacturing yield rate for obtaining the whole head is likely to decrease significantly due to the multiplication of the process yield for the light-source and the process yield for the head elements.
d) The head can be manufactured with reduced man-hour and at low cost, because of no need to provide the head with optical components such as a lens or prism which are required to have much high accuracy, or with optical elements having a special structure for connecting optical fibers or the like.
In fabrication of a thermally-assisted magnetic recording head having such a “composite slider structure”, it is significantly important to accurately align the light source unit with the slider when joining them together.
In practice, the head need to be fabricated in such a way that light emitted from the light-emission center located in the light-emitting surface of the light source is reliably allowed to be incident exactly at the light-receiving end of an optical system such as a waveguide located on the back surface of the slider in order to provide a sufficiently high light use efficiency. To this end, the light-emission center and the light-receiving end are aligned with each other in the track width direction and in the direction perpendicular to the track width direction as accurately as possible. Typically, it is preferable that the accuracy of the alignment be within ±1 μm (micrometer) in actual manufacturing.
One approach to achieving such high alignment accuracy is active alignment. In the active alignment, a light source such as a laser diode is actually being activated while the light source and the optical system are moved relative to each other, light emitted from the light source and incident at the light-receiving end of the optical system is monitored on the light-emitting end side of the optical system in real time, and a monitoring position at which the highest light intensity is obtained is set as the desired relative position of the light source and the optical system. However, the active alignment is a method of merely locating a two-dimensional optimum position and has the drawback of requiring a considerably long time for alignment. In addition, power supply probes need to be applied to the electrodes of the light source in order to keep activating the light source during the alignment, which further increase the time required for the alignment. Furthermore, a head structure and probing facilities which are required for the probing increase the manufacturing load.
There is another approach called passive alignment. In the passive alignment, a light source and an optical system are physically coupled to each other or are moved through image recognition, thus to align them with each other using an existing groove, an existing projection, or a marker provided in the light source and/or the optical system as a mark for alignment. In general, the passive alignment takes a shorter time than the active alignment. However, the accuracy of the passive alignment tends to be low compared with the active alignment. In addition, it is considerably difficult to find or add a marker for the passive alignment on the light-source unit during fabrication of a head having the “composite slider structure”.
In practice, in the “composite slider structure”, the alignment target in the light source unit is the light-emission center located in the light-emitting surface of the light source. If the light source is an edge-emitting laser diode, an end of a ridge structure located at the light-emitting surface of the diode is as small as approximately 2×2 μm2, for example, and is difficult to observe. Furthermore, even if the end can be observed, it is extremely difficult to identify the light-emission center in the end of the ridge structure. It may be contemplated to provide a marker for passive alignment on the light-emitting surface. However, provision of a light source, such as a laser diode, to which a maker suitable for image recognition is given will significantly increase manufacturing cost.
Therefore, there is a need for a novel alignment method capable of aligning a light source unit and a slider with each other with a sufficiently high alignment accuracy in a short processing time in fabrication of a thermally-assisted magnetic recording head having a “composite slider structure”.